Add missing methods to the Preferences API

Android SharedPreferences can handle a `java.util.Set[String]`, in
addition to the other types. This updates the `Preferences` class to
take a scala `Set[String]` and perform the appropriate conversions. In
addition, the `remove` method was missing from the API, and it has been
added.
Merge pull request #63 from sgrif/sg-complete-preferences-api

Add missing methods to the Preferences API